1. Field of the Invention
The present invention relates to a device which provides a conduit for patient respiratory gases and allows the passage of infrared radiation from a gas analyzer through the gases, and more particularly, to a disposable airway adapter for use with a respiratory gas analyzer.
2. Brief Description of the Prior Art
Respiratory gas analyzers function by passing light of a specific wavelength (typically infrared) through a gas and measuring the amount of light that is absorbed. Such a respiratory gas analyzer is disclosed by Solomon in an article entitled "A Reliable, Accurate CO.sub.2 Analyzer For Medical Use," Hewlett-Packard Journal, September 1981, Pages 3-21, for example. Solomon therein describes the HP CO.sub.2 Analyzer Model 47210A Capnometer, which measures the amount of carbon dioxide in a patient's breath for medical diagnostic purposes. The HP Model 47210A Capnometer is comprised of an airway adapter, a sensor and a processor box. The airway adapter, a hollow aluminum casting with sapphire windows, is inserted in series with the ventilator plumbing and is used to keep the patient's respiratory gases from coming in contact with the sensor mechanism. The sensor is snapped over the airway adapter windows, and the measurement is made directly on the artificial airway through which the patient is breathing. The sensor contains all the optical components necessary to make the infrared measurement and is connected to the processor box by a cable. The processor box powers the sensor, processes the return signal, and presents the data via an LED display.
Solomon states that the sensor of the HP Model 47210A Capnometer must be small, rugged, light-weight, and easily cleaned, and that the sensor must help isolate the processor box from any high voltages caused by the use of defibrillation equipment. The airway adapter also must be rugged and light-weight. It must be sterilizable and the infrared path length must be stable and consistent from unit to unit to minimize the total system error. Mating the airway adapter to standard ventilation plumbing also must be simple and reliable. In fact, the airway adapter of the HP Model 47210A Capnometer must satisfy a number of critical requirements. For example, sterilizability, a stable infrared path length and ruggedness dictate a series of materials requirements. Moreover, since measurement accuracy is directly related to the infrared path length through the sample, any variation from the nominal 3-mm gap results in an error proportional to the difference in the gap from the nominal value. Thus, to achieve the required stability in view of the other requirements, the airway adapter of the HP Model 47210A Capnometer is made of aluminum.
Aluminum is a preferred material for the airway adapter of the HP Model 47210A Capnometer since aluminum can withstand sufficiently high temperatures to allow sterilization of the airway adapter The aluminum airway adapter of the HP Model 47210A Capnometer also has two sapphire windows that are epoxy bonded to each side of the aluminum gas passage such that the gap between the windows forms a precise path length for the gas sample. The gap is set to a desired value during assembly by placing a shim of the correct thickness between the two windows and firmly clamping the windows to the shim while the epoxy bond cures, thereby forming the gap.
An airway adapter so formed allows accurate CO.sub.2 readings to be made for diagnostic purposes. However, as noted above, the HP Model 47210A Capnometer is made of investment cast aluminum while the windows are made of polished sapphire disks. The resulting adapter is thus too expensive to be disposable and hence must be sterilizable for multiple patient uses. A less expensive, and hence disposable, airway adapter is desired which maintains comparable accuracy in measurement.
It is generally known that medical devices may be made disposable for sanitary purposes by forming the devices of plastics such as polypropylene or polyethylene. For example, O'Hara, et al. disclose in U.S. Pat. No. 4,662,360 a disposable speculum which functions as a sanitary protective cover or sheath for an ear canal probe of a tympanic thermometer for measuring temperature using infrared energy. In particular, O'Hara, et al. disclose that polypropylene and polyethylene may be used as an infrared transparent membrane since both plastic materials are substantially transparent to infrared radiation at the wavelengths necessary for the infrared temperature measurement. Such an infrared transparent membrane must be relatively thin to minimize the attenuation of infrared radiation passing therethrough so that a thermopile or other detector receiving infrared radiation through the membrane will sense the maximum amount of infrared radiation available. The membrane should also have a uniform thickness, with no wrinkles or other structural characteristics that will distort the infrared radiation passing therethrough, for such distortion can introduce errors in the temperature measurement process. Accordingly, the membrane in O'Hara et al's preferred embodiment of a disposable speculum serving as an infrared window is made of polypropylene or polyethylene film having a thickness in the range of 0.0005 to 0.001 inch. Preferably, the speculum also mates with the probe of the tympanic thermometer so that the membrane is stretched tightly over the probe tip, thereby removing any wrinkles in the membrane.
O'Hara, et al. also disclose that the speculum is preferably manufactured by injection molding the entire speculum in one integral piece. However, since it is difficult to integrally mold the entire speculum with the walls and the membrane having thicknesses in the desired ranges, O'Hara, et al. disclose a preferred method of fabrication including injection molding of the tubular body portion of the speculum and then affixing a separate membrane to the frontal end of the body portion. A portion of the film defining the membrane thus is severed from a larger film and thermally bonded to the tubular body portion. This technique is relatively inexpensive and is conducive to mass production.
Thus, the device of O'Hara, et al. contains a thin infrared transmitting window for the altogether different application of measuring temperature via non-contact thermometry. Namely, the O'Hara, et al. device is used for separating the sensor probe of a tympanic thermometer from a patient's ear while taking temperatures. This device is manufactured by melt welding a thin polypropylene or polyethylene film onto a molded polypropylene or polyethylene body piece. Accordingly, although the resulting disposable speculum may be manufactured quite inexpensively, the device of O'Hara, et al. does not meet the structural requirements noted by Solomon for the manufacture of an airway adapter for use with a respiratory gas analyzer.
Raemer discloses in U.S. Pat. No. 4,648,396 a respiration detector having an infrared source and detector pair disposed on opposite sides of a cuvette through which the gas stream inhaled and exhaled by a patient is passed. The amount of CO.sub.2 in the exhaled stream is compared with that in the inhaled stream. Where the difference is greater than a predetermined amount, a breath is determined to have been detected, thus avoiding the necessity of calibrating the apparatus against any absolute reference standard. In a preferred embodiment of the respiration detector of Raemer, the elements of the sensor assembly are enclosed in a molded plastic housing with plastic windows. The gas cuvette also comprises plastic windows, preferably formed integrally, of a material which is relatively transparent to infrared radiation, such as polycarbonate plastics. In other words, Raemer specifies that both the sensor assembly and the cuvette have plastic windows. However, since the device of Raemer merely functions to determine whether the patient is breathing and not to determine absolute concentrations of constituents in the exhaled gases, measurement accuracy and hence calibration is not necessary. In fact, the Y-shaped adapter device of Raemer is disposed too far away from the patient's mouth to allow quantitative readings for determining the concentration of constituents in the exhaled air. Thus, the adapter device of Raemer, although plastic, could not function as a disposable airway adapter for use with a respiratory gas analyzer.
Accordingly, the inventors of the present invention know of no suitable prior art disposable airway adapter and believe there is a need in the art for an airway adapter for use with an infrared-type respiratory gas analyzer whereby the airway adapter has the suitable optical properties for allowing the infrared energy to pass through the gases yet is not prohibitively expensive. Preferably, such a device would be inexpensive enough that it may be disposable and hence need not be repeatedly sterilized to prevent the spread of disease and the like. The present invention has been designed to meet these needs.